diff --git a/lib/check.h b/lib/check.h
index 1a6c57faa..e98d1c7e3 100644
--- a/lib/check.h
+++ b/lib/check.h
@@ -61,10 +61,12 @@ public:
     /** This constructor is used when registering the CheckClass */
     explicit Check(const std::string &aname);
 
+protected:
     /** This constructor is used when running checks. */
     Check(std::string aname, const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : mTokenizer(tokenizer), mSettings(settings), mErrorLogger(errorLogger), mName(std::move(aname)) {}
 
+public:
     virtual ~Check() {
         if (!mTokenizer)
             instances().remove(this);
@@ -127,9 +129,9 @@ public:
         return false;
     }
 
+protected:
     static std::string getMessageId(const ValueFlow::Value &value, const char id[]);
 
-protected:
     const Tokenizer* const mTokenizer{};
     const Settings* const mSettings{};
     ErrorLogger* const mErrorLogger{};
diff --git a/lib/check64bit.h b/lib/check64bit.h
index 122fd8cb0..7ce317890 100644
--- a/lib/check64bit.h
+++ b/lib/check64bit.h
@@ -41,10 +41,13 @@ class Token;
  */
 
 class CPPCHECKLIB Check64BitPortability : public Check {
+    friend class Test64BitPortability;
+
 public:
     /** This constructor is used when registering the Check64BitPortability */
     Check64BitPortability() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     Check64BitPortability(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -58,8 +61,6 @@ public:
     /** Check for pointer assignment */
     void pointerassignment();
 
-private:
-
     void assignmentAddressToIntegerError(const Token *tok);
     void assignmentIntegerToAddressError(const Token *tok);
     void returnIntegerError(const Token *tok);
diff --git a/lib/checkassert.h b/lib/checkassert.h
index 7697f650d..4ddb8e156 100644
--- a/lib/checkassert.h
+++ b/lib/checkassert.h
@@ -41,9 +41,12 @@ class Token;
  */
 
 class CPPCHECKLIB CheckAssert : public Check {
+    friend class TestFixture;
+
 public:
     CheckAssert() : Check(myName()) {}
 
+private:
     CheckAssert(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
 
@@ -55,11 +58,9 @@ public:
 
     void assertWithSideEffects();
 
-protected:
     void checkVariableAssignment(const Token* assignTok, const Scope *assertionScope);
     static bool inSameScope(const Token* returnTok, const Token* assignTok);
 
-private:
     void sideEffectInAssertError(const Token *tok, const std::string& functionName);
     void assignmentInAssertError(const Token *tok, const std::string &varname);
 
diff --git a/lib/checkautovariables.h b/lib/checkautovariables.h
index b02a0d0a6..fa4b1d92f 100644
--- a/lib/checkautovariables.h
+++ b/lib/checkautovariables.h
@@ -45,10 +45,13 @@ namespace ValueFlow {
 
 
 class CPPCHECKLIB CheckAutoVariables : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckClass */
     CheckAutoVariables() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckAutoVariables(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -76,7 +79,6 @@ public:
 
     void checkVarLifetimeScope(const Token * start, const Token * end);
 
-private:
     void errorAutoVariableAssignment(const Token *tok, bool inconclusive);
     void errorReturnDanglingLifetime(const Token *tok, const ValueFlow::Value* val);
     void errorInvalidLifetime(const Token *tok, const ValueFlow::Value* val);
diff --git a/lib/checkbool.h b/lib/checkbool.h
index 94d1714a9..3c185095c 100644
--- a/lib/checkbool.h
+++ b/lib/checkbool.h
@@ -39,10 +39,13 @@ class Token;
 /** @brief checks dealing with suspicious usage of boolean type (not for evaluating conditions) */
 
 class CPPCHECKLIB CheckBool : public Check {
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckBool() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckBool(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -95,7 +98,6 @@ public:
     /** @brief %Check if a function returning bool returns an integer other than 0 or 1 */
     void returnValueOfFunctionReturningBool();
 
-private:
     // Error messages..
     void comparisonOfFuncReturningBoolError(const Token *tok, const std::string &expression);
     void comparisonOfTwoFuncsReturningBoolError(const Token *tok, const std::string &expression1, const std::string &expression2);
diff --git a/lib/checkboost.h b/lib/checkboost.h
index 376b137d5..1aba5b6d5 100644
--- a/lib/checkboost.h
+++ b/lib/checkboost.h
@@ -38,10 +38,13 @@ class Token;
 
 /** @brief %Check Boost usage */
 class CPPCHECKLIB CheckBoost : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckClass */
     CheckBoost() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckBoost(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -58,7 +61,6 @@ public:
     /** @brief %Check for container modification while using the BOOST_FOREACH macro */
     void checkBoostForeachModification();
 
-private:
     void boostForeachError(const Token *tok);
 
     void getErrorMessages(ErrorLogger *errorLogger, const Settings *settings) const override {
diff --git a/lib/checkbufferoverrun.h b/lib/checkbufferoverrun.h
index 16e3a7532..f83fd928b 100644
--- a/lib/checkbufferoverrun.h
+++ b/lib/checkbufferoverrun.h
@@ -57,11 +57,14 @@ class Token;
  * other function and pass a buffer and reads or writes too much data.
  */
 class CPPCHECKLIB CheckBufferOverrun : public Check {
-public:
+    friend class TestBufferOverrun;
+    friend class TestFixture;
 
+public:
     /** This constructor is used when registering the CheckClass */
     CheckBufferOverrun() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckBufferOverrun(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -97,8 +100,6 @@ public:
     /** @brief Analyse all file infos for all TU */
     bool analyseWholeProgram(const CTU::FileInfo *ctu, const std::list<Check::FileInfo*> &fileInfo, const Settings& settings, ErrorLogger &errorLogger) override;
 
-private:
-
     void arrayIndex();
     void arrayIndexError(const Token* tok,
                          const std::vector<Dimension>& dimensions,
diff --git a/lib/checkclass.h b/lib/checkclass.h
index 3134ebf15..a6b6732a2 100644
--- a/lib/checkclass.h
+++ b/lib/checkclass.h
@@ -51,10 +51,18 @@ namespace tinyxml2 {
 
 /** @brief %Check classes. Uninitialized member variables, non-conforming operators, missing virtual destructor, etc */
 class CPPCHECKLIB CheckClass : public Check {
+    friend class TestClass;
+    friend class TestConstructors;
+    friend class TestUnusedPrivateFunction;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckClass() : Check(myName()) {}
 
+    /** @brief Set of the STL types whose operator[] is not const */
+    static const std::set<std::string> stl_containers_not_const;
+
+private:
     /** @brief This constructor is used when running checks. */
     CheckClass(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger);
 
@@ -191,10 +199,6 @@ public:
     /** @brief Analyse all file infos for all TU */
     bool analyseWholeProgram(const CTU::FileInfo *ctu, const std::list<Check::FileInfo*> &fileInfo, const Settings& settings, ErrorLogger &errorLogger) override;
 
-    /** @brief Set of the STL types whose operator[] is not const */
-    static const std::set<std::string> stl_containers_not_const;
-
-private:
     const SymbolDatabase* mSymbolDatabase{};
 
     // Reporting errors..
diff --git a/lib/checkcondition.h b/lib/checkcondition.h
index 4c75a8a93..d1e29574f 100644
--- a/lib/checkcondition.h
+++ b/lib/checkcondition.h
@@ -48,10 +48,13 @@ namespace ValueFlow {
  */
 
 class CPPCHECKLIB CheckCondition : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckAssignIf */
     CheckCondition() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckCondition(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -128,7 +131,6 @@ public:
     /** @brief Assignment in condition */
     void checkAssignmentInCondition();
 
-private:
     // The conditions that have been diagnosed
     std::set<const Token*> mCondDiags;
     bool diag(const Token* tok, bool insert=true);
diff --git a/lib/checkexceptionsafety.h b/lib/checkexceptionsafety.h
index f70573c4b..e0a56c440 100644
--- a/lib/checkexceptionsafety.h
+++ b/lib/checkexceptionsafety.h
@@ -45,10 +45,13 @@ class Token;
  */
 
 class CPPCHECKLIB CheckExceptionSafety : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckClass */
     CheckExceptionSafety() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckExceptionSafety(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -88,7 +91,6 @@ public:
     /** @brief %Check for rethrow not from catch scope */
     void rethrowNoCurrentException();
 
-private:
     /** Don't throw exceptions in destructors */
     void destructorsError(const Token * const tok, const std::string &className);
     void deallocThrowError(const Token * const tok, const std::string &varname);
diff --git a/lib/checkfunctions.h b/lib/checkfunctions.h
index 05502a870..a4cdd4fe8 100644
--- a/lib/checkfunctions.h
+++ b/lib/checkfunctions.h
@@ -49,10 +49,14 @@ namespace ValueFlow {
  */
 
 class CPPCHECKLIB CheckFunctions : public Check {
+    friend class TestFunctions;
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckFunctions */
     CheckFunctions() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckFunctions(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -109,7 +113,6 @@ public:
     /** @brief --check-library: warn for unconfigured function calls */
     void checkLibraryMatchFunctions();
 
-private:
     /** @brief %Check for missing "return" */
     void checkMissingReturn();
 
diff --git a/lib/checkinternal.h b/lib/checkinternal.h
index e827d3c4e..823d3d2f5 100644
--- a/lib/checkinternal.h
+++ b/lib/checkinternal.h
@@ -39,10 +39,12 @@ class Token;
 
 /** @brief %Check Internal cppcheck API usage */
 class CPPCHECKLIB CheckInternal : public Check {
+    friend class TestFixture;
 public:
     /** This constructor is used when registering the CheckClass */
     CheckInternal() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckInternal(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -82,7 +84,7 @@ public:
 
     /** @brief %Check if there is a redundant check for none-nullness of parameter before Match functions, such as (tok && Token::Match(tok, "foo")) */
     void checkRedundantTokCheck();
-private:
+
     void multiComparePatternError(const Token *tok, const std::string &pattern, const std::string &funcname);
     void simplePatternError(const Token *tok, const std::string &pattern, const std::string &funcname);
     void complexPatternError(const Token *tok, const std::string &pattern, const std::string &funcname);
diff --git a/lib/checkio.h b/lib/checkio.h
index a1fdcd106..8fb109283 100644
--- a/lib/checkio.h
+++ b/lib/checkio.h
@@ -40,10 +40,13 @@ class ErrorLogger;
 
 /** @brief %Check input output operations. */
 class CPPCHECKLIB CheckIO : public Check {
+    friend class TestIO;
+
 public:
     /** @brief This constructor is used when registering CheckIO */
     CheckIO() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckIO(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -70,7 +73,6 @@ public:
     /** @brief %Checks type and number of arguments given to functions like printf or scanf*/
     void checkWrongPrintfScanfArguments();
 
-private:
     class ArgumentInfo {
     public:
         ArgumentInfo(const Token *arg, const Settings *settings, bool _isCPP);
diff --git a/lib/checkleakautovar.h b/lib/checkleakautovar.h
index 06243f513..8b44b915e 100644
--- a/lib/checkleakautovar.h
+++ b/lib/checkleakautovar.h
@@ -107,10 +107,13 @@ public:
  */
 
 class CPPCHECKLIB CheckLeakAutoVar : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckLeakAutoVar */
     CheckLeakAutoVar() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckLeakAutoVar(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -120,8 +123,6 @@ public:
         checkLeakAutoVar.check();
     }
 
-private:
-
     /** check for leaks in all scopes */
     void check();
 
diff --git a/lib/checkmemoryleak.h b/lib/checkmemoryleak.h
index ca2adde63..c406ffccf 100644
--- a/lib/checkmemoryleak.h
+++ b/lib/checkmemoryleak.h
@@ -163,11 +163,14 @@ public:
  * -# finally, check if the simplified token list contain any leaks.
  */
 
-class CPPCHECKLIB CheckMemoryLeakInFunction : private Check, public CheckMemoryLeak {
+class CPPCHECKLIB CheckMemoryLeakInFunction : public Check, public CheckMemoryLeak {
+    friend class TestMemleakInFunction;
+
 public:
     /** @brief This constructor is used when registering this class */
     CheckMemoryLeakInFunction() : Check(myName()), CheckMemoryLeak(nullptr, nullptr, nullptr) {}
 
+private:
     /** @brief This constructor is used when running checks */
     CheckMemoryLeakInFunction(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger), CheckMemoryLeak(tokenizer, errorLogger, settings) {}
@@ -182,7 +185,6 @@ public:
      */
     void checkReallocUsage();
 
-private:
     /** Report all possible errors (for the --errorlist) */
     void getErrorMessages(ErrorLogger *e, const Settings *settings) const override {
         CheckMemoryLeakInFunction c(nullptr, settings, e);
@@ -217,10 +219,13 @@ private:
  * @brief %Check class variables, variables that are allocated in the constructor should be deallocated in the destructor
  */
 
-class CPPCHECKLIB CheckMemoryLeakInClass : private Check, private CheckMemoryLeak {
+class CPPCHECKLIB CheckMemoryLeakInClass : public Check, private CheckMemoryLeak {
+    friend class TestMemleakInClass;
+
 public:
     CheckMemoryLeakInClass() : Check(myName()), CheckMemoryLeak(nullptr, nullptr, nullptr) {}
 
+private:
     CheckMemoryLeakInClass(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger), CheckMemoryLeak(tokenizer, errorLogger, settings) {}
 
@@ -234,7 +239,6 @@ public:
 
     void check();
 
-private:
     void variable(const Scope *scope, const Token *tokVarname);
 
     /** Public functions: possible double-allocation */
@@ -262,10 +266,13 @@ private:
 
 /** @brief detect simple memory leaks for struct members */
 
-class CPPCHECKLIB CheckMemoryLeakStructMember : private Check, private CheckMemoryLeak {
+class CPPCHECKLIB CheckMemoryLeakStructMember : public Check, private CheckMemoryLeak {
+    friend class TestMemleakStructMember;
+
 public:
     CheckMemoryLeakStructMember() : Check(myName()), CheckMemoryLeak(nullptr, nullptr, nullptr) {}
 
+private:
     CheckMemoryLeakStructMember(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger), CheckMemoryLeak(tokenizer, errorLogger, settings) {}
 
@@ -276,8 +283,6 @@ public:
 
     void check();
 
-private:
-
     /** Is local variable allocated with malloc? */
     bool isMalloc(const Variable *variable) const;
 
@@ -298,10 +303,13 @@ private:
 
 /** @brief detect simple memory leaks (address not taken) */
 
-class CPPCHECKLIB CheckMemoryLeakNoVar : private Check, private CheckMemoryLeak {
+class CPPCHECKLIB CheckMemoryLeakNoVar : public Check, private CheckMemoryLeak {
+    friend class TestMemleakNoVar;
+
 public:
     CheckMemoryLeakNoVar() : Check(myName()), CheckMemoryLeak(nullptr, nullptr, nullptr) {}
 
+private:
     CheckMemoryLeakNoVar(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger), CheckMemoryLeak(tokenizer, errorLogger, settings) {}
 
@@ -312,7 +320,6 @@ public:
 
     void check();
 
-private:
     /**
      * @brief %Check if an input argument to a function is the return value of an allocation function
      * like malloc(), and the function does not release it.
diff --git a/lib/checknullpointer.h b/lib/checknullpointer.h
index e3b7c2714..1786f9804 100644
--- a/lib/checknullpointer.h
+++ b/lib/checknullpointer.h
@@ -47,32 +47,13 @@ namespace tinyxml2 {
 /** @brief check for null pointer dereferencing */
 
 class CPPCHECKLIB CheckNullPointer : public Check {
+    friend class TestNullPointer;
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckNullPointer */
     CheckNullPointer() : Check(myName()) {}
 
-    /** @brief This constructor is used when running checks. */
-    CheckNullPointer(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
-        : Check(myName(), tokenizer, settings, errorLogger) {}
-
-    /** @brief Run checks against the normal token list */
-    void runChecks(const Tokenizer &tokenizer, ErrorLogger *errorLogger) override {
-        CheckNullPointer checkNullPointer(&tokenizer, tokenizer.getSettings(), errorLogger);
-        checkNullPointer.nullPointer();
-        checkNullPointer.arithmetic();
-        checkNullPointer.nullConstantDereference();
-    }
-
-    /**
-     * @brief parse a function call and extract information about variable usage
-     * @param tok first token
-     * @param var variables that the function read / write.
-     * @param library --library files data
-     */
-    static void parseFunctionCall(const Token &tok,
-                                  std::list<const Token *> &var,
-                                  const Library *library);
-
     /**
      * Is there a pointer dereference? Everything that should result in
      * a nullpointer dereference error message will result in a true
@@ -86,6 +67,29 @@ public:
 
     static bool isPointerDeRef(const Token *tok, bool &unknown, const Settings *settings);
 
+private:
+    /**
+     * @brief parse a function call and extract information about variable usage
+     * @param tok first token
+     * @param var variables that the function read / write.
+     * @param library --library files data
+     */
+    static void parseFunctionCall(const Token &tok,
+                                  std::list<const Token *> &var,
+                                  const Library *library);
+
+    /** @brief This constructor is used when running checks. */
+    CheckNullPointer(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
+        : Check(myName(), tokenizer, settings, errorLogger) {}
+
+    /** @brief Run checks against the normal token list */
+    void runChecks(const Tokenizer &tokenizer, ErrorLogger *errorLogger) override {
+        CheckNullPointer checkNullPointer(&tokenizer, tokenizer.getSettings(), errorLogger);
+        checkNullPointer.nullPointer();
+        checkNullPointer.arithmetic();
+        checkNullPointer.nullConstantDereference();
+    }
+
     /** @brief possible null pointer dereference */
     void nullPointer();
 
@@ -117,7 +121,6 @@ public:
     /** @brief Analyse all file infos for all TU */
     bool analyseWholeProgram(const CTU::FileInfo *ctu, const std::list<Check::FileInfo*> &fileInfo, const Settings& settings, ErrorLogger &errorLogger) override;
 
-private:
     /** Get error messages. Used by --errorlist */
     void getErrorMessages(ErrorLogger *errorLogger, const Settings *settings) const override {
         CheckNullPointer c(nullptr, settings, errorLogger);
diff --git a/lib/checkother.h b/lib/checkother.h
index 35aad044e..b1969a47d 100644
--- a/lib/checkother.h
+++ b/lib/checkother.h
@@ -47,14 +47,27 @@ class ErrorLogger;
 /** @brief Various small checks */
 
 class CPPCHECKLIB CheckOther : public Check {
+    friend class TestCharVar;
+    friend class TestIncompleteStatement;
+    friend class TestOther;
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckOther() : Check(myName()) {}
 
+    /** Is expression a comparison that checks if a nonzero (unsigned/pointer) expression is less than zero? */
+    static bool comparisonNonZeroExpressionLessThanZero(const Token *tok, const ValueFlow::Value **zeroValue, const Token **nonZeroExpr);
+
+    /** Is expression a comparison that checks if a nonzero (unsigned/pointer) expression is positive? */
+    static bool testIfNonZeroExpressionIsPositive(const Token *tok, const ValueFlow::Value **zeroValue, const Token **nonZeroExpr);
+
+private:
     /** @brief This constructor is used when running checks. */
     CheckOther(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
 
+
     /** @brief Run checks against the normal token list */
     void runChecks(const Tokenizer &tokenizer, ErrorLogger *errorLogger) override {
         CheckOther checkOther(&tokenizer, tokenizer.getSettings(), errorLogger);
@@ -103,13 +116,6 @@ public:
         checkOther.checkOverlappingWrite();
     }
 
-    /** Is expression a comparison that checks if a nonzero (unsigned/pointer) expression is less than zero? */
-    static bool comparisonNonZeroExpressionLessThanZero(const Token *tok, const ValueFlow::Value **zeroValue, const Token **nonZeroExpr);
-
-    /** Is expression a comparison that checks if a nonzero (unsigned/pointer) expression is positive? */
-    static bool testIfNonZeroExpressionIsPositive(const Token *tok, const ValueFlow::Value **zeroValue, const Token **nonZeroExpr);
-
-
     /** @brief Clarify calculation for ".. a * b ? .." */
     void clarifyCalculation();
 
@@ -229,7 +235,6 @@ public:
     void overlappingWriteUnion(const Token *tok);
     void overlappingWriteFunction(const Token *tok);
 
-private:
     // Error messages..
     void checkComparisonFunctionIsAlwaysTrueOrFalseError(const Token* tok, const std::string &functionName, const std::string &varName, const bool result);
     void checkCastIntToCharAndBackError(const Token *tok, const std::string &strFunctionName);
diff --git a/lib/checkpostfixoperator.h b/lib/checkpostfixoperator.h
index 99460cb31..2c163ecfc 100644
--- a/lib/checkpostfixoperator.h
+++ b/lib/checkpostfixoperator.h
@@ -40,10 +40,13 @@ class Token;
  */
 
 class CPPCHECKLIB CheckPostfixOperator : public Check {
+    friend class TestPostfixOperator;
+
 public:
     /** This constructor is used when registering the CheckPostfixOperator */
     CheckPostfixOperator() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckPostfixOperator(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -59,7 +62,6 @@ public:
     /** Check postfix operators */
     void postfixOperator();
 
-private:
     /** Report Error */
     void postfixOperatorError(const Token *tok);
 
diff --git a/lib/checksizeof.h b/lib/checksizeof.h
index f47863c58..e738c8fce 100644
--- a/lib/checksizeof.h
+++ b/lib/checksizeof.h
@@ -39,10 +39,13 @@ class Token;
 /** @brief checks on usage of sizeof() operator */
 
 class CPPCHECKLIB CheckSizeof : public Check {
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckSizeof() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckSizeof(const Tokenizer* tokenizer, const Settings* settings, ErrorLogger* errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -86,7 +89,6 @@ public:
     /** @brief %Check for using sizeof(void) */
     void sizeofVoid();
 
-private:
     // Error messages..
     void sizeofsizeofError(const Token* tok);
     void sizeofCalculationError(const Token* tok, bool inconclusive);
diff --git a/lib/checkstl.h b/lib/checkstl.h
index 8ec207cfd..dffacfe8b 100644
--- a/lib/checkstl.h
+++ b/lib/checkstl.h
@@ -43,10 +43,13 @@ class ErrorLogger;
 
 /** @brief %Check STL usage (invalidation of iterators, mismatching containers, etc) */
 class CPPCHECKLIB CheckStl : public Check {
+    friend class TestFixture;
+
 public:
     /** This constructor is used when registering the CheckClass */
     CheckStl() : Check(myName()) {}
 
+private:
     /** This constructor is used when running checks. */
     CheckStl(const Tokenizer* tokenizer, const Settings* settings, ErrorLogger* errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -184,7 +187,6 @@ public:
 
     void checkMutexes();
 
-private:
     bool isContainerSize(const Token *containerToken, const Token *expr) const;
     bool isContainerSizeGE(const Token * containerToken, const Token *expr) const;
 
diff --git a/lib/checkstring.h b/lib/checkstring.h
index c5cf999c3..0b2cb4ac2 100644
--- a/lib/checkstring.h
+++ b/lib/checkstring.h
@@ -39,10 +39,13 @@ class Token;
 /** @brief Detect misusage of C-style strings and related standard functions */
 
 class CPPCHECKLIB CheckString : public Check {
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckString() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckString(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -82,7 +85,6 @@ public:
     /** @brief %Check for overlapping source and destination passed to sprintf() */
     void sprintfOverlappingData();
 
-private:
     void stringLiteralWriteError(const Token *tok, const Token *strValue);
     void sprintfOverlappingDataError(const Token *funcTok, const Token *tok, const std::string &varname);
     void strPlusCharError(const Token *tok);
diff --git a/lib/checktype.h b/lib/checktype.h
index c174691f6..dcab41b52 100644
--- a/lib/checktype.h
+++ b/lib/checktype.h
@@ -42,10 +42,13 @@ class ValueType;
 /** @brief Various small checks */
 
 class CPPCHECKLIB CheckType : public Check {
+    friend class TestFixture;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckType() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckType(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -77,8 +80,6 @@ public:
     void checkFloatToIntegerOverflow();
     void checkFloatToIntegerOverflow(const Token *tok, const ValueType *vtint, const ValueType *vtfloat, const std::list<ValueFlow::Value> &floatValues);
 
-private:
-
     // Error messages..
     void tooBigBitwiseShiftError(const Token *tok, int lhsbits, const ValueFlow::Value &rhsbits);
     void tooBigSignedBitwiseShiftError(const Token *tok, int lhsbits, const ValueFlow::Value &rhsbits);
diff --git a/lib/checkuninitvar.h b/lib/checkuninitvar.h
index ea9e75ed7..8ab6fd9b8 100644
--- a/lib/checkuninitvar.h
+++ b/lib/checkuninitvar.h
@@ -60,10 +60,18 @@ struct VariableValue {
 /** @brief Checking for uninitialized variables */
 
 class CPPCHECKLIB CheckUninitVar : public Check {
+    friend class TestUninitVar;
+
 public:
     /** @brief This constructor is used when registering the CheckUninitVar */
     CheckUninitVar() : Check(myName()) {}
 
+    enum Alloc { NO_ALLOC, NO_CTOR_CALL, CTOR_CALL, ARRAY };
+
+    static const Token *isVariableUsage(bool cpp, const Token *vartok, const Library &library, bool pointer, Alloc alloc, int indirect = 0);
+    const Token *isVariableUsage(const Token *vartok, bool pointer, Alloc alloc, int indirect = 0) const;
+
+private:
     /** @brief This constructor is used when running checks. */
     CheckUninitVar(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -80,15 +88,12 @@ public:
     void check();
     void checkScope(const Scope* scope, const std::set<std::string> &arrayTypeDefs);
     void checkStruct(const Token *tok, const Variable &structvar);
-    enum Alloc { NO_ALLOC, NO_CTOR_CALL, CTOR_CALL, ARRAY };
     bool checkScopeForVariable(const Token *tok, const Variable& var, bool* const possibleInit, bool* const noreturn, Alloc* const alloc, const std::string &membervar, std::map<nonneg int, VariableValue> variableValue);
     const Token* checkExpr(const Token* tok, const Variable& var, const Alloc alloc, bool known, bool* bailout = nullptr) const;
     bool checkIfForWhileHead(const Token *startparentheses, const Variable& var, bool suppressErrors, bool isuninit, Alloc alloc, const std::string &membervar);
     bool checkLoopBody(const Token *tok, const Variable& var, const Alloc alloc, const std::string &membervar, const bool suppressErrors);
     const Token* checkLoopBodyRecursive(const Token *start, const Variable& var, const Alloc alloc, const std::string &membervar, bool &bailout) const;
     void checkRhs(const Token *tok, const Variable &var, Alloc alloc, nonneg int number_of_if, const std::string &membervar);
-    static const Token *isVariableUsage(bool cpp, const Token *vartok, const Library &library, bool pointer, Alloc alloc, int indirect = 0);
-    const Token *isVariableUsage(const Token *vartok, bool pointer, Alloc alloc, int indirect = 0) const;
     static int isFunctionParUsage(const Token *vartok, const Library &library, bool pointer, Alloc alloc, int indirect = 0);
     int isFunctionParUsage(const Token *vartok, bool pointer, Alloc alloc, int indirect = 0) const;
     bool isMemberVariableAssignment(const Token *tok, const std::string &membervar) const;
@@ -130,7 +135,6 @@ public:
     }
     void uninitStructMemberError(const Token *tok, const std::string &membername);
 
-private:
     std::set<const Token*> mUninitDiags;
     Check::FileInfo* getFileInfo() const;
 
diff --git a/lib/checkunusedfunctions.cpp b/lib/checkunusedfunctions.cpp
index b1dff174d..fad92637e 100644
--- a/lib/checkunusedfunctions.cpp
+++ b/lib/checkunusedfunctions.cpp
@@ -49,10 +49,10 @@ namespace CTU {
 
 //---------------------------------------------------------------------------
 
-
-
 // Register this check class
-CheckUnusedFunctions CheckUnusedFunctions::instance;
+namespace {
+    CheckUnusedFunctions instance;
+}
 
 static const struct CWE CWE561(561U);   // Dead Code
 
@@ -68,6 +68,12 @@ static std::string stripTemplateParameters(const std::string& funcName) {
 // FUNCTION USAGE - Check for unused functions etc
 //---------------------------------------------------------------------------
 
+void CheckUnusedFunctions::clear()
+{
+    instance.mFunctions.clear();
+    instance.mFunctionCalls.clear();
+}
+
 void CheckUnusedFunctions::parseTokens(const Tokenizer &tokenizer, const char FileName[], const Settings *settings)
 {
     const bool doMarkup = settings->library.markupFile(FileName);
diff --git a/lib/checkunusedfunctions.h b/lib/checkunusedfunctions.h
index e0e58cf5b..762300ccf 100644
--- a/lib/checkunusedfunctions.h
+++ b/lib/checkunusedfunctions.h
@@ -44,6 +44,8 @@ namespace CTU {
 /// @{
 
 class CPPCHECKLIB CheckUnusedFunctions : public Check {
+    friend class TestUnusedFunctions;
+
 public:
     /** @brief This constructor is used when registering the CheckUnusedFunctions */
     CheckUnusedFunctions() : Check(myName()) {}
@@ -52,10 +54,7 @@ public:
     CheckUnusedFunctions(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
 
-    static void clear() {
-        instance.mFunctions.clear();
-        instance.mFunctionCalls.clear();
-    }
+    static void clear();
 
     // Parse current tokens and determine..
     // * Check what functions are used
@@ -71,15 +70,12 @@ public:
     /** @brief Analyse all file infos for all TU */
     bool analyseWholeProgram(const CTU::FileInfo *ctu, const std::list<Check::FileInfo*> &fileInfo, const Settings& settings, ErrorLogger &errorLogger) override;
 
-    static CheckUnusedFunctions instance;
-
     std::string analyzerInfo() const;
 
     /** @brief Combine and analyze all analyzerInfos for all TUs */
     static void analyseWholeProgram(const Settings &settings, ErrorLogger * const errorLogger, const std::string &buildDir);
 
 private:
-
     void getErrorMessages(ErrorLogger *errorLogger, const Settings * /*settings*/) const override {
         CheckUnusedFunctions::unusedFunctionError(errorLogger, emptyString, 0, "funcName");
     }
diff --git a/lib/checkunusedvar.h b/lib/checkunusedvar.h
index 843704f20..0275c5530 100644
--- a/lib/checkunusedvar.h
+++ b/lib/checkunusedvar.h
@@ -45,10 +45,13 @@ class Function;
 /** @brief Various small checks */
 
 class CPPCHECKLIB CheckUnusedVar : public Check {
+    friend class TestUnusedVar;
+
 public:
     /** @brief This constructor is used when registering the CheckClass */
     CheckUnusedVar() : Check(myName()) {}
 
+private:
     /** @brief This constructor is used when running checks. */
     CheckUnusedVar(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
@@ -69,7 +72,6 @@ public:
     /** @brief %Check that all struct members are used */
     void checkStructMemberUsage();
 
-private:
     bool isRecordTypeWithoutSideEffects(const Type* type);
     bool isVariableWithoutSideEffects(const Variable& var);
     bool isEmptyType(const Type* type);
diff --git a/lib/checkvaarg.h b/lib/checkvaarg.h
index e2f0ad928..2284e2bff 100644
--- a/lib/checkvaarg.h
+++ b/lib/checkvaarg.h
@@ -40,9 +40,12 @@ class Token;
  */
 
 class CPPCHECKLIB CheckVaarg : public Check {
+    friend class TestFixture;
+
 public:
     CheckVaarg() : Check(myName()) {}
 
+private:
     CheckVaarg(const Tokenizer *tokenizer, const Settings *settings, ErrorLogger *errorLogger)
         : Check(myName(), tokenizer, settings, errorLogger) {}
 
@@ -55,7 +58,6 @@ public:
     void va_start_argument();
     void va_list_usage();
 
-private:
     void wrongParameterTo_va_start_error(const Token *tok, const std::string& paramIsName, const std::string& paramShouldName);
     void referenceAs_va_start_error(const Token *tok, const std::string& paramName);
     void va_end_missingError(const Token *tok, const std::string& varname);